CLI config refactor and improvements

[swalkinshaw]

Adds a new cli_config package which allows for setting CLI config values from multiple files and ENV variables. The precedence from lowest to highest is:

1. global config
2. project config
3. env variables

The global CLI config (defaults to $HOME/.config/trellis/cli.yml) and will be loaded first on boot (if it exists).

Next the project CLI config will be loaded if a projected is detected ($PROJECT/.trellis/cli.yml).

Finally, env variables prefixed with TRELLIS_ will be used as overrides if they match a supported configuration setting.

Current supported settings:

Setting	Description	Type	Default
ask_vault_pass	Set Ansible to always ask for the vault pass	boolean	false
check_for_updates	Whether to check for new versions of trellis-cli	boolean	true
load_plugins	Load external CLI plugins	boolean	true
open	List of name -> URL shortcuts	map[string]string	none
virtualenv_integration	Enable automated virtualenv integration	boolean	true

Example config:

ask_vault_pass: false
check_for_updates: true
load_plugins: true
open:
  site: "https://mysite.com"
  admin: "https://mysite.com/wp/wp-admin"
virtualenv_integration: true

Example env var usage:

TRELLIS_ASK_VAULT_PASS=true trellis provision production